The object detecting device for an automatic door system or for security monitoring purpose is generally so designed that while a detection wave in the form of a microwave is emitted towards a detection area from which the presence or absence of an object such as, for example, one or more human bodies is desired to be detected, the object can be detected in terms of a change in frequency brought about by the Doppler effect when a portion of the detection wave reflected from the object is received.
As an antenna for defining the detection area for such an object detecting device, a horn antenna has hitherto been employed largely. However, since the horn antenna unit including the horn antenna itself and peripheral parts represents a shape generally similar to the shape of a trumpet and is therefore bulky in size, the use of the horn antenna tends to result in increase in size of the object detecting device and, therefore, the handling of the object detecting device as a whole during the installation thereof tends to be complicated and time-consuming.
Also, it is a general practice that depending on the environment in which the object detecting device is set up, the detection area that can be covered by the object detecting device is adjusted to a relatively wide detection area where the object detecting device is set up in a door assembly of a relatively large width, to a relatively narrow detection area where the object detecting device is set up in a door assembly of a relatively small width. With the standard object detecting device utilizing the horn antenna, the adjustment of the detection area is carried out by moving adjustable walls, which are disposed on left and right sides of the horn antenna, respectively, for the adjustment of the coverage of the object detecting device, or by installing the adjustable walls at predetermined or required positions, followed by turning the horn antenna unit through an angle of 90°. Such procedures for adjusting the detection area are indeed complicated and time-consuming.
In order to alleviate such problems and inconveniences as discussed above, the object detecting device assembled compact in size and designed to facilitate the adjustment of the detection area has been suggested, in which a patch antenna prepared from a metallic foil is employed in place of the horn antenna discussed above. See, for example, the Japanese Laid-open Patent Publication No. 7-110375. According to this prior art, a dielectric lens, which may become a concave lens or a convex lens when turned 90°, is disposed rotatably in front of the patch antenna, such that the detection area can be easily and smartly adjusted to one of the wide and narrow detection areas when the dielectric lens is turned 90° in either direction.
However, even in the object detecting device utilizing the rotatable dielectric lens discussed above, since only one lens is turned to accomplish the adjustment of the detection area, the width to which the detection area can be adjusted is limited and it may often occur that no sufficient adjustment can be achieved.
In the case of the object detecting device, it is often observed that the microwave radiation beam emitted from the object detecting device contains small beam components, generally referred to as side lobes, on respective sides of a primary beam component intermediate between those side lobes. The presence of those side lobes means that some of the output of the primary beam component is lost to the side lobes, accompanied by reduction of the output of the primary beam to such an extent that unnecessary detection areas, which ought not to be formed, tend to be formed simultaneously with formation of the detection area defined by the primary beam. If those unnecessary detection areas are formed in the manner described above, and where the object detecting device is employed as a sensor for selectively detecting opening or closure of an automatic door, the object detecting device will detect a human body or any other object present within any of the unnecessary detection areas, posing the possibility that the automatic door will unnecessarily be closed or opened even though the automatic door ought not to be opened or closed. In addition, the detection area available includes two types, i.e., wide and narrow areas and no area intermediate between those wide and narrow areas can be formed. Therefore, the prior art is insufficient in that a variety of detection areas to cope with a variety of environments for installation of the object detection device cannot be available.